Airway oxygen monitoring, or oxygraphy, is used in anesthesia and critical care situations to provide an indication of oxygen delivery to and utilization (i.e. oxygen consumption) by the patient. The difference between inspired and end-tidal oxygen fraction is useful to determine, for example, the amount of oxygen extraction which serves as a measure of cardiac and pulmonary function (e.g. adequacy of perfusion and metabolism) and overall physiologic condition of the patient. Oxygen consumption is commonly used to monitor the fitness or physiological condition of an individual or athlete. The phrases “oxygen update” and “oxygen consumption” are used synonymously, and are both represented by the expression “{dot over (V)}O2,” or, for simplicity “VO2”. Oxygen consumption is a measure of the amount of oxygen that the body uses in a given period of time, such as one minute. It is typically expressed as milliliters of oxygen used per kilogram of body weight per minute (ml/kg/min), simply in milliliters of oxygen used per minute.
Traditionally, oxygraphy is accomplished using a side-stream gas sampling system. In a sidestream monitoring system, a gas sample is taken from a sample site, such as the patient's airway via a nasal cannula or a patient circuit through a gas sampling line, to a sensing mechanism or sample cell that is located some distance from the sample site for monitoring. A drying system is typically included in the cannula, sample cell, or sampling line so that sidestream flow of gas entering the sample cell is relatively moisture free. If the drying system consists of a section of Nafion tubing, the gas sample is dried to ambient humidity. Similarly, the transport of the gas through the sampling line results in the temperature of the sample equilibrating to the ambient temperature prior to analysis by the sensor. For these reasons, both inspired and expired gas, are analyzed in side-stream monitoring systems as if the gas was at ambient temperature and humidity.
When oxygraphy is performed using an on-airway oxygen sensor (i.e., a mainstream gas sensor in which all or most the gas delivered to or received from the patient passes through the sample site), the gas being analyzed will likely vary in both temperature and humidity. The expired gas is nearly always 100% saturated (relative humidity=100%) and at body temperature or slightly below body temperature. On the other hand, the inspired gas may be actively heated and humidified using a vaporizer, may be passively humidified using a heat-moisture exchanger, or may be at ambient conditions. In any case, it is unlikely that the intra-airway temperature and humidity data will be available to the oxygen monitor.
As gas is humidified and water vapor is added to the gas, the oxygen in the gas is diluted and the concentration of oxygen in the gas decreases. If inspired gas is dry and expired gas is humidified, the oxygraph measured by an on-airway (mainstream) oxygen sensor will show a difference in inspired and expired oxygen just based on the changes of oxygen concentration due to warming and/or humidification. While the oxygen concentration measurement is physically accurate, it is clinically misleading. Even though there is an actual difference in inspired and expired oxygen fraction, this difference could be misinterpreted as an indication of patient perfusion and metabolism, rather than simple gas warming and humidification.
VO2 is conventionally calculated as the difference between the volume of oxygen inspired and the volume of oxygen expired. The standard or direct calculation of VO2 is given by the following equation:VO2=Vi*FiO2−Ve*FēO2,  (1)where: “VO2” is oxygen consumption, “Vi” is inspired volume, “FiO2” is the inspired oxygen concentration, “Ve” is the expired volume, and “FēO2” is the mixed expired oxygen concentration. An error occurs in this calculation if the expired gas has been heated and moistened by the lungs, and the inspired gas is cooler and/or drier than the expired gas. The effect of heating and/or humidification means that the expired volume (Ve) will be larger than the inspired volume (Vi) and the measured expired oxygen fraction (FēO2) will be lower than the actual oxygen fraction, leading to a falsely large VO2 determination. Ideally, if it were possible to measure the inspired oxygen fraction (FiO2) and inspired volume (Vi) under known temperature and humidity conditions, then the direct VO2 calculation would be accurate despite the differences in temperature and humidity between inspired and expired gasses.
This direct calculation of oxygen consumption described in equation (1) is simple and valid, but it can lead to errors in the calculated VO2 in situations where there are small errors in the gas volume measurement, i.e., the measurement of Vi, Ve, or both. Gas temperature and humidity differences are a major source of these inspired-expired gas volume differences. This problem is exacerbated at high oxygen concentrations.
An alternative method of calculating VO2 uses only the expired breath volume, Ve. In this scenario, the inspired breath volume Vi is calculated (rather than measured) based on the assumption that, on average, the nitrogen volume is the same for both inspired and expired gas, which is usually true because nitrogen is not consumed or produced by the body. This is referred to as the nitrogen balance. The calculation of Vi, rather than measuring it, also assumes that the effect of temperature and humidity are the same for both inspired and expired gas volumes.
This modification of equation (1), which uses a calculation of Vi based on the nitrogen balance noted above, is known as the Haldane transform. According to this technique, Vi is calculated as follows:Vi=Ve*FēN2/FiN2  (2)Where “FēN2” is the concentration of mixed expired nitrogen, and “FiN2” is the concentration of mixed inspired nitrogen. The nitrogen concentration can be calculated as the balance gas (or gas that is neither oxygen or CO2, both of which are directly measured) assuming that the only gases in the airways are oxygen, carbon dioxide, and nitrogen. Based on this, the Haldane transform becomes:Vi=Ve*(1−FēCO2−FēO2)/(1−FiCO2−FiO2),  (3)and the oxygen consumption calculation becomes:VO2=Ve*[FiO2*((1−FēCO2−FēO2)/(1−FiCO2−FiO2))−FēO2],  (4)where FēCO2 is the mixed expired carbon dioxide concentration, and FiCO2 is the mixed inspired carbon dioxide concentration.
Calculating VO2 using the Haldane transform has the advantage that the effects of errors in volume measurements that are not “common mode” are eliminated, because only the expired volume measurement is used. Common mode errors are errors that affect both the Vi and Ve measurements, such as a calibration error in a flow sensor. Assuming, of course, the same sensor is used to measure Ve and Vi.
As noted above, expired volume is often larger than inspired volume because the exhaled gas is warmed and humidified by the lungs. When the Haldane transform is used, the added volume due to temperature and humidity causes an invalid estimation of Vi when the FiO2 is measured in dry gas, which leads to an erroneously high calculated Vi and VO2.
This is typically not a problem when a conventional side-stream gas sampling system is used to measure FiO2, FēO2, FiCO2, and FēCO2, because conventional side-stream gas sampling systems typically include a gas drying system, as noted above.
If, however, a mainstream monitoring system is used to measure FiO2, FēO2, FiCO2, and FēCO2, the use of the inspired-expired temperature and humidity differences can lead to an error. In a mainstream monitoring system, the sampling site is located in-situ in a patient circuit or conduit coupled to the patient's airway. As a result, the patient's expired gases are normally saturated with water vapor and have a temperature of about 35° C. If the inspired gas is cooler and/or drier than the expired gas, errors result in the VO2 calculation using either the direct or Haldane transform equations. Using the direct calculation method, it would be necessary to correct both the inspired oxygen fraction (FiO2) and the inspired volume (Vi) to expired gas temperature and humidity conditions to make a correct calculation. Using the Haldane transform, only the inspired oxygen fraction must be corrected to the same warm and wet conditions as seen in expired gas for the calculation to be valid.